According to well-known optical principles, when a lens focuses light from objects which are located at different distances from the lens, the image produced of each such object on the opposite side of the lens has its sharpest focus at a different distance from the lens. In traditional cameras in which a photodetector, such as an area of photographic film or an array of electronic sensors, is placed at one given distance from the lens, this creates a problem because it is impossible for the one distance of the photodetector from the lens to coincide with the different distance from the lens at which the images of different objects are in sharpest focus. This phenomenon has given rise to the concept of depth of field.
The depth of field of a camera defines the range of distance from the camera at which different objects may be located and still be focused with clarity in the same image. In traditional cameras, depth of field results from the fact that although a given portion of an image is in sharpest focus only at one specific distance from the focusing lens, it is in relatively clear focus over a range of distance on both sides of that specific distance, and the greater the depth of field is the greater that range is. Thus even through the distance from a lens at which two separate objects are in sharpest focus may differ, if there is a distance from the lens at which their images are both in relatively clear focus, a photodetector, such as a piece of film, placed at that distance can obtain a clear image of both objects.
The depth of field of an optical system is determined by well-known optical laws which relate to factors such as the F-stop of the lens used in such a system, the brightness of the objects being focused, the sensitivity of the film or photodetectors sensing the focused image, the motion of the image, and lens quality. Thus, when attempting to focus objects placed at different distances from a lens, a depth of field sufficient to produce a clear image of all such objects can often be achieved by using bright scene illumination which enables a camera to be operated at a high F-number. However, there are many situations in which it is not possible to achieve a desired depth of field with traditional cameras. For example, one of the objects to be focused may be dimly lit and too far away to permit adequate illumination to operate a camera at an F-number which would provide the depth of field necessary to focus both it and the other nearer objects. Even when it is physically possible to provide sufficient illumination to operate with an adequate F-number, it may not be desirable to do so since such illumination may consume considerable energy, may require bulky equipment, may produce unpleasant heat and brightness, and may make it difficult to photograph a scene without drawing attention.